Always split collections and argument lists with trailing commas.

test/splitting/arguments.stmt

 40 columns                              |
 >>> many arguments
 method(first, second, third, fourth, fifth, sixth, seventh, eighth, ninth,
     tenth, eleventh, twelfth);
 <<<
 method(
     first,
     second,
     third,
     fourth,
@@ skipping 183 matching lines @@
     ],
     argument,
     {
       'key': value,
       'other key': value,
       'third key': value
     }, () {
   ;
 });
 >>> trailing comma
-functionn(argument,argument  ,argument  ,  );
+fn(argument,argument  ,argument  ,  );
 <<<
-functionn(
-    argument, argument, argument,);
+fn(
+  argument,
+  argument,
+  argument,
+);
 >>> trailing comma in named argument list
-function(named: argument,another:argument,  );
+fn(named: argument,another:argument,  );
 <<<
-function(
-    named: argument,
-    another: argument,);
+fn(
+  named: argument,
+  another: argument,
+);